Gaseous discharge device



May 12, 1970 P. w. STUTSMAN 3,51

GASEOUS DISCHARGE DEViCE Filed Nov. 28, 1967 0 3C OQOOOO OOOOOO O o O O o o O INVENTOR PAUL W STUTSMAN' United States Patent US. Cl. 313-346 9 Claims ABSTRACT OF THE DISCLOSURE A gaseous discharge flashtube having improved life and low noise, comprising a gas-filled envelope containing anode, cathode and trigger electrodes wherein the cathode is formed partially of barium-aluminum eutectic alloy, and an apertured ceramic disc having holes therein of at least two different sizes is included within the envelope for absorbing high frequency shock waves produced by rapid changes in gas density resulting from sudden heating of the gas plasma when arcing occurs between the cathode and anode electrodes.

Background of the invention This invention relates to gaseous-discharge devices and has particular reference to improved high intensity gaseous-discharge flashtubes of a type useful to produce illumination in character display apparatus, stroboscopic systems, flash photography, and the like.

It is highly desirable in such systems and apparatus that a small, compact light source be provided to develop illumination of relatively high intensity such as twentyfive watts, for example, in the form of a single pulse or a series of pulses of such illumination. A conventional light-source of this type is described in US. Pat. Nos. 2,492,142 and 2,977,508, for example, and comprises a. glass envelope filled with gas, such as xenon, and containing a number of spaced trigger electrodes located between. a pair of main electrodes. With voltage just below the breakdown voltage applied across the cathode and anode, the trigger may be pulsed to a selected frequency to cause a primary discharge to sectionally ionize a small cross-section path terminating at the main electrodes and connecting the ends of the trigger electrodes.

When this path is energized, a shunt capacity between the main electrodes will dump its energy, causing an arc discharge of several hundred peak amperes and of high peak luminous intensity along the ionized path.

In such prior art lamps or light sources the cathode electrodes are composed of tungsten and nickel metal powders mixed with barium peroxide or aluminate, which mix is pressed into pellets of the desired form and fired in hydrogen. However, it has been found that the life of such cathodes in lamps of this character are relatively short, the cathodes exhibiting deterioration after operation for only 2000 hours or less, caused by excessive erosion of the cathode which led to blackening of the bulb and to a rise in the required trigger voltage. By

2000 hours, this voltage often rises to a point where the lamps will not operate in some display systems, and at 2500 hours these lamps sometimes went into continuous conduction.

Attempts have been made to overcome the erratic tendency of prior art lamps to go into continuous conduction by adding a small quantity of hydrogen (H to the xenon fill. This trace of H additive will reduce the deionizing time but, as life progresses, the H is cleaned up and the beneficial aspects of the additive are lost.

Further in such prior art lamps, the plasma region is intensely heated when the arc occurs, causing the gas density in that region to change rapidly. This produces,

a shock Wave which communicates its energy to the lamp walls and, hence, to the surrounding air as noise. In many cases, the noise increases with power input, and this is also true with added gas pressures. Such shock waves often cause cracks to develop in the glass envelopes, particularly in envelopes having portions of different glass compositions.

Summary of the invention The foregoing and other disadvantages of and objections to the prior art are overcome in the present invention by the provision of lamps having cathodes .made of barium-aluminum (BaAl) eutectic alloy, tungsten (W) and nickel (Ni) pressed into pellet form and fired in hydrogen at temperatures ranging from about 800 C. to about 1900" C., preferably about 950 C. During the firing, the metals of the pellet become transparent to hydrogen, and the barium in the BaAl alloy reacts with hydrogen to form barium hydride. After firing, the cathode experiences a maximum temperature of about 350 C. during vacuum processing. Thus, the barium is present in the finally fabricated lamp as a hydride and as such may serve as a continuous hydrogen dispenser during subsequent lamp operation. Consequently, traces of hydrogen are present in the xenon gas filling and render considerably less pronounced the lamps tendency to go into continuous conduction.

A further improvement is achieved by the reduction or elimination of objectionable noise by the provision within the lamp of an internal noise absorber. This absorber is a ceramic disc having numerous holes, preferably of two or more different sizes. Such discs have been found to be effective in reducing at least the high frequency noise component which is the most objectionable. The disc must be of a highly refractory type ceramic which can be subjected without deterioration to the heat produced by the arc.

Brief description of the drawings The foregoing and other objectives of this invention are achieved by the lamp disclosed in the attached drawings, wherein:

FIG. 1 is a perspective view of a gaseous-discharge flashtube embodying the present invention, with part of the glass envelope being broken away to clearly disclose details of the interior construction; and

FIG. 2 is a perspective view of a ceramic :noise suppression disc utilized in the device shown in FIG. 1.

Description of the preferred embodiment The flashtube of lamp 10 as shown in FIG. 1 comprises a light transparent envelope or bulb 12 of glass or the like which contains the selected gaseous medium, preferably xenon or combinations of xenon and krypton. The envelope is provided with a tubulation 14 through which the gas may be introduced into the interior during tube fabrication and processing, after which the tubulation is pinched off to seal the gas within the envelope. The gas may be maintained at any suitable pressure such as, for example, nineteen to twenty-five inches, it being understood that gas pressure is one of the several variables which effect the peak light intensity produced by the arc in xenon, and that higher gas pressures are allowable with shorter arc paths.

Within the envelope 12 are an anode electrode 16 and a cathode electrode 18, both being preferably of similar construction, which are mounted in encircling conductive supports 20 and 22, respectively, in electrical communication with or On pin terminals 24 and 26, respectively, extending through and sealed within the envelope base portion 28. These electrodes are spaced apart at a distance which is selected to be the desired length of the arc to be generated between the electrodes, such distance also being ;elected in accordance with the utilized gas pressure. Ex- :ellent results have been obtained with tuibes having mode-cathode spacings of .22l-.320 inch, for example.

The anode 16 and cathode 18, in accordance with this nvention are made of barium-aluminum (BaAl) eutectic alloy mixed with tungsten (W) and nickel (Ni), all in aowder form in various percentage mixtures. The Ba is rlloyed with the Al in an inert atmosphere and the re- ;ultant ingot is'then ground to a powder and mixed with :he W and Ni powders, and the mixture subsequently aressed into pellet form and fired in hydrogen (H at :emperatures ranging from about 800 C. to about 1900" 3. Excellent results are obtained using about 83% W, about 11.9% Ni, about 2.45% Al, and about 2.65% Ba, ired at about 950 C. for about five minutes.

It has been found that pure metal cathodes of nickel, .ungsten, thorium, etc. are rapidly eroded and, conseuently, lamp life may be limited to one hundred hours )r less. A nickel matrix with BaAl alloy gives much imarovement. Still further gains, to be set forth hereinafter, '6 made with a matrix of tungsten (W) with sutficient Ii to bind the W at 950 C. in H Also to obtain improvement over prior art cathodes, the 3aAl-W-Ni mixture may contain an additive such as strontium (Sr) or calcium (Ca). When Sr is used as the rdditive, one suggested formula comprises approximately be following: 83% W, 11.9% Ni, 2.45% Al, 1.59% Ba and 1.06% Sr. Another formula comprises approximately :he following: 83.0% W, 11.9% Ni, 2.0% A1, 1.59% Ba, l.06% Sr and 0.45% Ca.

The pellets formed in accordance with the above are nounted in the encircling supports 20-22, as shown in FIG. 1, with substantially flat rectangular surfaces paral- .el to and facing one another. Such electrodes are capable f withstanding the high temperatures inherent in the Jperation of closely spaced electrodes at substantial voltages in a high-pressure gas.

A number of probe-type trigger or control electrodes 30a-30b-30c-30d and 302 extend substantially normal to :he axis of the tube in spaced relation with one another in the space between the facing surfaces of the anode and :athode electrodes. These trigger electrodes are substan- ;ially rod-shaped members preferably formed of tungsten )r the like and are supported by pin-type trigger terminals 2a32b-32c32d and 32e sealed in and extending through envelope base 28. While five such trigger electrodes are shown, it is to be understood that a greater or lesser lumber thereof may be employed consistent with the spacing between the main anode and cathode electrodes and the control desired of the arc to be produced between the main electrodes. An additional trigger electrode 34 is located in the lower regions of the envelope, being mount- :d on the inner end of another trigger terminal 36 and :onductively connected by connection 38 to the cathode :erminal 26. This trigger electrode 34 is provided for assisting in the ionization of gas in the lower portion of :he envelope.

Resting upon the base portion 28 within the envelope 12 is a ceramic disc 40, preferably alumina, which contains openings through which the various terminals extend. Disc 40 is retained in position by any suitable means such as a pin or the like (not shown) mounted in base 28, for example, and is provided with a relatively large number at holes, preferably of two or more diametric sizes, for the purpose of suppressing internally generated noise, as will be described hereinafter.

In the operation of a lamp of this character, a voltage is applied between the cathode and anode which is insufficient to produce a discharge therebetween through the gas. A trigger impulse is then applied to break down the gas in the neighborhood of either the anode 16 or the :athode 18, between the same and the adjacent trigger electrode 30a or Me, respectively. Assuming that, for example, breakdown is first initiated between the trigger electrode 30c and the cathode 18, an arc passes therebetween, dropping the potential of the trigger electrode to that of the cathode 18. By Providing sufiicient isolating impedance between the trigger electrode 302 and the next adjacent trigger electrode 30d, the potential of the trigger electrode 30d does not drop to the cathode potential at the instant of the discharge between the cathode 18 and trigger electrode 302, but remains at a high potential with respect thereto. The space between the cathode-potential trigger electrode 302 and the adjacent trigger electrode 30d thus next becomes broken down. In a similar manner, successive breakdowns will occur in series between trigger electrode 30d and trigger electrode 300, then between trigger electrode 300 and trigger electrode 30b, then between trigger electrode 30b and trigger electrode 30a, and finally between trigger electrode 30a and the anode 16. The voltage applied between the anode 16 and the cathode 18 can thus become discharged in the space between them, producing a high intensity flash. In view of the series breakdown, the discharge takes place along a short path controlled by the trigger electrodes within the volume defined by the space between the anode and the cathode.

With a structure as shown in FIG. 1, a flashing rate of about 2500 per second is possible, with the flash intensity being extremely high throughout the life of the lamp, reaching from 280 to 350 E0 (-peak mv.). The associated equipment, such as power supplies, circuits, etc., necessary to effect operation of a tube of the character described is conventional and well known and, therefore, is not described herein. Aforementioned U.S. Patents 2,492,142 and 2,977,508 may be referred to for additional information in this respect.

It has been found that shock waves from the are discharge produce noise or buifeting which sometimes produces a crack or fracture between portions of the envelope which may be formed of different glass compositions, for example, the bulb glass which is usually made of lime glass and the stem or base portion which is usually made of lead glass. When the arc occurs, as described above, the plasma region is intensely heated, causing the gas density in that region to change rapidly. This produces shock waves which communicate their energy to the lamp walls and hence to the surrounding air as noise. Therefore, in accordance with this invention there is provided the internal noise absorber disc 40 to suppress at least the high frequency noise component which is most objectionable. The disc 40 is -a highly refractory type ceramic which will withstand the heat produced by the are, such as alumina ceramic. Such a disc preferably has a large number of holes 42 (FIG. 2) of at least two difierent sizes to aid in noise suppression.

While discs or layers of other materials might be employed in some cases as suppressors, fibrous elements such as quartz cloth have proved generally unsatisfactory because small fibers become detached by shock wave impact and may be come vaporized in the arc and eventually deposited upon the walls of the bulb as a film which reduces light transmission.

Flashtubes embodying the present invention have been operated satisfactorily for from about 4000 to about 7500 hours or more without deterioration such as many times occurs after less than 2000 hours with tubes having cathodes constructed in accordance with the prior art.

Accordingly, all of the objectives of this invention have been achieved by the novel tube or lamp shown and described. However, it is to be understood that various changes in the general structure of the tube may be made such as, for example, changing the number, sizes, shapes, locations or spacings of the trigger electrodes, the shapes and sizes of the main electrodes, and the amounts of the various materials comprising the cathode compositions, without departing from the spirit of the invention as ex pressed in the appended claims.

What is claimed is:

1. A long-life gaseous-discharge device having a gas-.

filled envelope within the gas of which are disposed spaced anode and cathode electrodes defining an arc path therebetween, and a plurality of spaced trigger electrodes having end portions terminating in the space between the anode and cathode electrodes for controlling an are passing along the arc path between said anode and cathode electrodes, at least one of said anode and cathode electrodes being a pellet comprising a composition containing a mixture of barium-aluminum eutectic alloy, tungsten and nickel.

2. A gaseous discharge device as set forth in claim 1 wherein said one of said electrodes is the cathode.

3. A gaseous discharge device as set forth in claim 1 wherein said composition includes about 83% W, about 11.9% Ni, about 2.45% Al, and about 2.65% Ba.

4. A gaseous discharge device as set forth in claim 1 wherein said mixture further includes an additive selected from the group consisting or strontium and calcium and combinations thereof.

5. A gaseous discharge device as set forth in claim 1 wherein the envelope also contains noise suppressor means for absorbing shock waves caused by the are passing between the anode and cathode electrodes, said suppressor means being a disc of refractory type material having therein a plurality of open apertures of various sizes.

6. A gaseous discharge device as set forth in claim 5 wherein the envelope has walls transparent to light, and the noise supppressor is located in the envelope in a position which will not interfere substantially with the passage of light from the are through the walls of the en- Velope.

References Cited UNITED STATES PATENTS 2,121,589 6/1938 Espe 313-346 X 2,492,142 12/ 1949 Germeshausen 313-346 X 2,577,809 12/1951 Reeves et a1 313-260 X 2,724,216 11/1955 Cisne 313-260 X 2,835,843 5/1958 Millis 313-260 X 3,350,602 10/ 1967 Germeshausen 315-241 FOREIGN PATENTS 653,592 5/ 1951 Great Britain. 899,233 12/ 1953 Germany.

JOHN W. HUCKERT, Primary Examiner A. I. JAMES, Assistant Examiner US. Cl. X.R. 

